Transition metal catalyst complexes play important roles in many areas of chemistry, including the preparation of polymers and pharmaceuticals. The properties of these catalyst complexes are recognized to be influenced by both the characteristics of the metal and those of the ligands associated with the metal atom. For example, structural features of the ligands can influence reaction rate, regioselectivity, and stereoselectivity. Bulky ligands can be expected to slow reaction rate; electron-withdrawing ligands, in coupling reactions, can be expected to slow oxidative addition to, and speed reductive elimination from, the metal center; and electron-rich ligands, in coupling reactions, conversely, can be expected to speed oxidative addition to, and slow reductive elimination from, the metal center.
In many cases, the oxidative addition step in the accepted mechanism of a coupling reaction is deemed to be rate limiting. Therefore, adjustments to the catalytic system as a whole that increase the rate of the oxidative addition step should increase overall reaction rate. Additionally, the rate of oxidative addition of a transition metal catalyst to the carbon-halogen bond of an aryl halide is known to decrease as the halide is varied from iodide to bromide to chloride, all other factors being equal. Because of this fact, the more stable, lower molecular weight, and arguably more easy to obtain, members of the set of reactive organic halides—the chlorides—are the poorest substrates for traditional transition metal catalyzed coupling reactions and the like.
To date, the best halogen-containing substrates for transition metal catalyzed carbon-heteroatom and carbon-carbon bond forming reactions have been the iodides. Bromides have often been acceptable substrates, but typically required higher temperatures, longer reaction times, and gave lower yields of products.